The flow induced vibration of air filled in a housing in which a magnetic disk and a magnetic head are accommodated, is considered as one factor causing vibration of the magnetic disk and the magnetic head or a reading/writing error which occurs in a magnetic disk drive such as a hard disk drive device or the like.
Therefore, it has been proposed that gas (helium or the like) having lower density than air is filled in the housing of the magnetic disk drive. However, such low-density gas is more liable to leak to the outside of the housing of the magnetic disk drive as compared with air, and thus it is necessary to estimate variation of the amount of the gas in the housing.
Therefore, it has been proposed to judge the concentration variation of helium or the like filled in the housing of the magnetic disk drive on the basis of the amplitude of a reading signal, the running current of a spindle motor or the like, for example, Japanese Patent Publication No. 2004-535647 (“Patent Document 1”).
However, in the above conventional technique, it is necessary to make the judgment under the state that the rotation of the spindle motor reaches a stationary state and the normal reading/writing operation of the magnetic disk is enabled, and thus there is a problem in convenience.